In many applications it is desirable to have graphite impregnated in or coated on a polymeric substrate. The graphite supplies lubricity and conductivity to the composite. Packings and seals are an area where the composite of graphite and a polymer is particularly advantageous.
A packing is a sealing member which is used to prevent or minimize leakage between two components of a fluid container or a pump, which components may undergo relative motion. The packing should fit correctly in the packing space, be able to withstand the temperature and pressure conditions, be negligibly affected by the fluid being sealed and be sufficiently flexible to accommodate the varying degrees of longitudinal and/or radial displacement.
Where there is relatively no motion, slow or infrequent motion, such as in valves, packings can seal without leakage. When rapid motion occurs, either reciprocating or rotating, such as in centrifugal pumps, packings must permit the escape of continuous small amounts of liquid past the seal. Continuous, effective operation is dependent on the maintenance of wetted surfaces to minimize heat build up.
A common type of shaft seal consists of a packing composed of fibers or filaments which are first woven, twisted or braided into strands and then formed into coils, spirals or rings for insertion around the shaft.
Many packings have an impregnant built into them which, among other things, acts as a primary lubricant for the start up and break-in adjustment. During this period, a certain amount of the primary lubricant is lost. A large portion of the present braided packings tend to leak excessively. This results in the primary lubricant bleeding out, causing friction with the attendant generation of heat, which is the primary cause of packing failure.
The break-in period of the packings requires careful adjustment of the compression force against the packing and several such adjustments may be required. Otherwise, the packings either leak excessively or, when too much compression is used, overheat and score the rotating shafts of the seal so they must be replaced.
Because of its chemical and biological inertness and its lubricity, polytetrafluoroethylene (PTFE) has found wide application as a packing material in the chemical, food and drug industry.
A disadvantage of the use of PTFE as a pump packing is that it is a good insulator and does not conduct away the heat generated during operation as rapidly as some other packings. An attempt was made to overcome this problem by impregnating the PTFE with a good heat conductive material which is also a lubricant. The most frequently used impregnant for this purpose is graphite. While graphite impregnation results in improved heat transfer, problems have arisen in that some particles of impregnant graphite break free from the host PTFE packing and contaminate the fluid being transported. These graphite impregnated packings also suffer from the drawback that they are messy and dirty to handle.
In the early 1970's a new form of PTFE became available. This form of PTFE was unique in that it combined high porosity with high strength. The process for manufacturing this unique product is described in U.S. Pat. No. 3,953,566. A continuation-in-part, U.S. Pat. No. 3,962,153 discloses very high strength porous filaments which are particularly useful for braiding, weaving or twisting into packing. The products of U.S. Pat. No. 3,962,153 can be impregnated with graphite and overcome a degree of the problems outlined above. They do not offer a completely satisfactory solution, however.